Galileo Galilei 1610
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Figure 2
History of
Discovery
Le Opera di Galileo Galilei Edizione Nazionale
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Istituto e Museo di Storia della Scienza
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Italian astronomer (1564–1642)
In 1610, Galileo Galilei observed Saturn through a refractor
telescope, which used lenses a few inches across to magnify distant objects by 20 to 30 times. He observed Saturn to be
“triple,” thinking that he was seeing two “lesser stars” (moons)
or companions to the planet, or perhaps they were two bulges
that were features of the main body. In the fall of 1612 he observed Saturn again and to his surprise he found the planet to
be perfectly round! In 1616, he found that Saturn’s “companions” had reappeared and grown, and he made a drawing that
today would be readily interpreted as Saturn and its ring system. Galileo died never knowing that he’d been the first to observe Saturn’s rings.
(Middle) Galileo’s 1610 drawing of Saturn. His telescope was unable to resolve the
ring shape and he thought he was seeing three objects.
(Left) Galileo’s 1616 drawing of Saturn. He was able to discern the ring shape but
thought it was attached to the planet.
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Christiaan Huygens 1655
Figure 3
Dutch astronomer (1629–1695)
History of
Discovery
The Museon, Le Hague
Huygensmuseum Hofwijck
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In March 1655, at age 26, Christiaan Huygens viewed Saturn
through a more powerful telescope than Galileo’s. Huygens’ skill
at crafting lenses allowed his instrument to magnify objects
about 50 times. Huygens’ observations and his study of other
astronomers’ observations led him to determine that Saturn had
a flat ring encircling its equator, and unlike Galileo, he could
see that the ring did not touch the planet. Huygens discovered
Saturn’s largest moon, which would be named Titan 200 years
later. He observed that the moon orbited Saturn every 16 days.
Its orbit was well beyond the extent of the ring, but like the
ring, it was in the plane of Saturn’s equator. He recognized that
the disappearance of Saturn’s “companions,” which Galileo had
observed in 1612, occurred whenever Saturn’s thin ring appeared edge-on to Earth observers. This event occurs about every 15 years because, like the Earth, Saturn’s north–south axis is
tilted. This tilt causes our view of Saturn’s rings to change as the
planet travels in its 30-year orbit around the Sun. The Huygens
probe to Titan, built by the European Space Agency, is named
after Christiaan Huygens.
(Bottom) Huygens’ drawing of Saturn, 1683.
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Jean-Dominique Cassini 1675
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Italian–French astronomer
(1625–1712)
In the late 17th century, Jean-Dominique Cassini studied Saturn
from the Paris Observatory using a series of refracting telescopes.
Cassini’s largest telescope was 136 feet long, dwarfing Galileo’s
4-foot telescope and allowing him to see far greater detail. In
1675, Cassini discovered that Saturn’s “solid” ring had a gap in it
that divided it into two separate rings. Today this gap is called
the Cassini Division. The ring outside the division is the A ring,
while the brighter ring within is the B ring. No one yet knew
what the rings were made of, how thick they were, or whether
they were permanent features around Saturn. Cassini also discovered four moons of Saturn (the moons weren’t named until
later): Iapetus (1671), Rhea (1672), and Tethys and Dione
(1684). Other moons would not be discovered until over a century later. Cassini is the astronomer for whom NASA’s Cassini
mission to Saturn is named.
Figure 4
History of
Discovery
Paris Observatory
Paris Observatory
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(Left) Many astronomers of Cassini’s time used tubeless telescopes. A framework
structure or a rope, as shown here, was used to align the eyepiece with the
objective lens.
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William Herschel 1781
Figure 5
German–British astronomer (1738–1822)
History of
Discovery
Royal Astronomical Society
Royal Astronomical Society
Yerkes Observatory
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In 1781, William Herschel discovered the planet Uranus beyond
the orbit of Saturn, ending Saturn’s long reign as the most-distant planet. The discovery also effectively doubled the size of the
Solar System because Uranus is twice as far away from the Sun as
is Saturn. In 1789, Herschel discovered the sixth and seventh
moons of Saturn, which would later be named Mimas and
Enceladus. Herschel was among the first to use a reflecting telescope, which used mirrors instead of lenses to focus the light
coming from distant objects. He constructed and used a telescope with a 48inch mirror that weighed a ton and was
housed in a tube 40 feet long. The telescope was located in his backyard in Bath,
England. To reach the eyepiece, he
climbed a scaffolding that rose 50 feet
into the air! Herschel was often assisted by
his sister Caroline, who was also an accomplished astronomer.
(Above) Caroline Herschel.
(Left) Herschel’s 40-foot-long reflector telescope in Bath, England.
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James Keeler 1895
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History of
Discovery
Mary Lea Shane Archives, Lick Observatory
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Mary Lea Shane Archives, Lick Observatory
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American astronomer (1857–1900)
In 1888, James Keeler observed Saturn with the telescope at the
Lick Observatory in California. On the first night the observatory began operating, Keeler saw a narrow, dark gap close to the
outer edge of Saturn’s A ring. This gap is now called the Encke
Gap (the astronomer Johann Encke received the credit, though
he could not quite resolve the gap in detail when he observed it
in 1837). In 1895, Keeler observed Saturn’s rings using a telescope at the Allegheny Observatory in Pennsylvania. Connected
to the telescope was a spectrograph, which analyzed the light
reflected from the rings. The light he saw indicated that the innermost parts of the rings were moving around Saturn faster
than the outermost parts. This offered experimental evidence
that the rings were not a solid disk, but instead made up of individual particles moving like tiny moons around Saturn.
(Left) An 1888 engraving of the 36-inch refractor used by Keeler at the Lick
Observatory.
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Gerard Kuiper 1944
Figure 7
Dutch–American astronomer (1905–1973)
History of
Discovery
McDonald Observatory
Yerkes Observatory
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In 1944, Gerard Kuiper [KOY-per] discovered Titan’s atmosphere using a spectrograph attached to a reflector telescope at
McDonald Observatory in Texas. Unlike Keeler’s spectrograph,
Kuiper’s spectrograph detected infrared light (that is, infrared
radiation, often called heat) instead of visible light. Kuiper was
particularly interested in finding out if any of the moons orbiting around other planets in the Solar System had atmospheres.
He studied the infrared light reflected off the 10 largest moons,
and in 1944 he reported that Titan, the largest moon of Saturn,
was the only one having an atmosphere that could be easily detected. Astronomers observed the sky only in visible light until
the 1930s, when the first radio-wavelength observations were made. Today, we view the Universe across the entire electromagnetic spectrum, in radio, microwave, infrared, visible,
ultraviolet, x-ray, and gamma ray.
(Left) This is the 82-inch reflector telescope at McDonald
Observatory that Kuiper used for his observations.
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Pioneer 11 1979
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U.S. spacecraft
NASA’s small robotic observer passed within 22,000 km of
Saturn’s cloudtops in September 1979, providing the first closeup images of the Saturn system. Pioneer 11 took pictures of
Saturn’s poles and clouds, detected heat generated deep from
within Saturn, made the first measurements of Saturn’s magnetic field, confirmed the E ring (suggested in 1967 by scientists studying Earth-based telescope images), discovered the
F ring just outside the A ring, and made a possible detection
of the G ring (a faint, narrow ring just beyond the F ring).
Pioneer 11’s view of Saturn was about 50,000 times closer than
any Earth-based telescope could see. Pioneer 11 represented a
new way for astronomers to explore the planets. Rather than
scientists building their own telescopes and working individually to make new discoveries from observatories on Earth, a
team of people — scientists, engineers, and different kinds of
specialists working together — built a robot having various
instruments and sent it into space to send back images and
other kinds of data.
Figure 8
History of
Discovery
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(Left) A portion of Saturn’s rings observed by Pioneer 11.
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Voyager 1 & 2 1980–1981
Figure 9
U.S. spacecraft
History of
Twin NASA spacecraft made extensive flyby
studies of Saturn — in November 1980,
Voyager 1 passed within 125,000 km of Saturn’s
cloudtops, and in August 1981, Voyager 2
passed within 101,000 km. The Voyager missions sent back tens of thousands of color images
of the Saturn system. They measured high wind
speeds along Saturn’s equator, provided close-up
pictures of several known moons, and discovered
that Titan’s atmosphere is very thick and made mostly of nitrogen. The Voyagers also discovered several small moons: Telesto,
Calypso, Pan, Atlas, Prometheus, and Pandora. Voyager cameras
showed us that Saturn’s rings are actually made up of thousands
of tiny “ringlets,” and that strange spoke-like structures hover
above the B ring. The spacecraft confirmed the existence of
both the innermost D ring as well as the outer G ring that had
been tentatively identified by Pioneer 11. By observing the way
radio waves and visible light pass through the rings, scientists
inferred from Voyager data that ring particles range in size from
nearly invisible dust to icebergs the size of houses.
Discovery
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(Bottom) A Voyager close-up image of Saturn’s rings.
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Hubble Space Telescope 1995
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History of
Discovery
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IMAX, Smithsonian, Lockheed
Figure 10
U.S. orbiting observatory
NASA launched the Earth-orbiting Hubble Space Telescope
(HST) in 1990 from Space Shuttle Discovery. HST’s main mirror is 2.4 meters across, and the entire telescope is about the size
of a school bus. HST observes in visible, ultraviolet, and infrared light. It is not significantly closer to the planets and stars
than are telescopes on the ground, but its views of the Universe
are undistorted by Earth’s atmosphere. HST orbits the planet
every 90 minutes from about 600 km (370 miles) above the
surface. Astronauts can visit it every few years to
upgrade the instruments. HST has provided
views of cloud eruptions in Saturn’s atmosphere,
monitored the thickness and density of the
faint, outermost E ring, and searched for new
Saturn moons during times when the rings appear edge-on to Earth observers. HST has also
made infrared images of Titan’s surface that helped to show
where the Cassini mission would land its Huygens probe.
(Top) Deploying the HST on April 25, 1990, from the Space Shuttle.
(Bottom) A nearly edge-on view of Saturn’s rings taken by the Wide-Field and
Planetary Camera aboard HST.
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Cassini–Huygens 2004–2008
Figure 11
U.S. spacecraft and European probe
History of
Cassini–Huygens was launched in 1997 and will arrive at Saturn in 2004. During a 4-year tour of the Saturn system, the
spacecraft will study the majestic planet, its extraordinary rings,
and its moons. The Cassini orbiter carries six instruments to
“see” in four kinds of “light” (visible, infrared, ultraviolet, and
radio), as well as instruments for measuring dust particles,
charged particles, and magnetic fields. The Huygens probe will
parachute through Titan’s atmosphere and land on the surface,
taking more than 1,000 images of Titan’s clouds and surface.
No human has ever seen the sights that will be captured by the
Huygens probe — will there be lakes, oceans, mountains, and
craters? Compared with the instruments aboard the Voyagers
or the Hubble Space Telescope, the Cassini orbiter instruments
can observe in much finer detail. Cassini will have 4 full years
to study the Saturn system instead of just a few days as did
the Pioneer 11 and Voyager flyby missions. Cassini will fly
within about 20,000 km of Saturn’s cloudtops, and as close
as 1,000 km to some of the moons.
Discovery
Card
(Top) Cassini–Huygens in the assembly bay at JPL.
(Left) Artist’s concept of the Huygens probe landing on Titan.
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